Research into the anaerobic degradation pathways of chlorinated ethenes, ethanes, and other anaerobically degradable contaminants has begun to evaluate the biotic and abiotic contributions to overall contaminant degradation. The idea of combined mechanisms has not been investigated as the remediation industry grew from two major approaches, a biological approach and a chemical oxidation approach.
In-situ bioremediation (ISB) of contaminants is practiced commercially and is effective on contaminants that are degraded by being reduced or oxidized. Biological processes mainly revolved around the application of a carbon substrate to enhance reductive dechlorination. The bioremediation of contaminants requires four components: microorganisms, electron donors, electron acceptors, and nutrients.
In the case of oxidative bioremediation, the contaminants are the electron donors and oxygen is the electron acceptor. Crude oil would be the best example of a contaminant that benefits from oxidative bioremediation.
For reductive bioremediation, the contaminants are the electron acceptors and an organic compound is the electron donor. Reductive bioremediation is applicable to halogenated compound, nitroamines, perchlorates, metals and other compounds that can be degraded, precipitated or immobilized by being reduced. Examples include (Reference: Handbook of Environmental Degradation Rates, Philip H. Howard, et al., 1991 by CRC Press LLC), but are not limited to: halogenated ethenes, halogenated ethanes, halogenated methanes, straight-chain halogenated hydrocarbons, halogenated aromatics, halogenated organic compounds, polycyclic halogenated compound such as polychlorinated biphenyls (PCB); nitrates, nitrites, sulfates; explosives, munitions, nitroaromatics; perchlorates, chlorates; dioxins, polychlorinated dibenzo-p-dioxins (PCDDs), polychlorinated dibenzofurans (PCDFs); methyl tertiary butyl ether (MTBE); pesticides, herbicides, insecticides, defoliants and other agricultural chemicals; N-nitrosodiumethylamine (NDMA); organic dyes (Orange III, Chrysoidin, Tropaeolin O, etc); radio nucleotides; metals including but not limited to copper, molybdenum, uranium, chromium, selenium, vanadium, arsenic, silver, antimony, cadmium, lead, mercury, thallium, tin, cobalt, iron, manganese, nickel, zinc, aluminum, gold, barium, radium, and magnesium.
Many different organic carbons have been used commercially to stimulate reductive bioremediation process including: volatile fatty acids and derivatives including sodium lactate, potassium lactate, ethyl lactate/dipropionate, citric acid; sugars including molasses, sucrose, glucose, fructose; oils including vegetable oil, emulsified vegetable oil, oil mixes, gasoline/diesel; polymerized poly-lactate; and solids including chitin, whey, wood mulch.
The one evolution was the use of emulsified vegetable oil as a common slow release substrate.
Chemical reduction of contaminants utilizing metals and metal minerals has also been used commercially for many years. Zero Valent Iron (ZVI) is the most widely used in-situ chemical reluctant (ISCR) of this type. Early incarnations of this technology utilized iron filings filled trenches to form permeable reactive barriers (PRBs). More recent examples include ZVI powder mixed with solid carbohydrates and nano and micro scale ZVI mixed with vegetable oil emulsions. There is also research on injecting soluble iron compounds and a chemical or biological reducing agent and forming metal minerals in-situ. All of these technologies use iron or iron minerals to reduce contaminant with abiotic processes. The main challenge with the introduction of an ISCR is a general inability to transport the material away from the injection points to ensure adequate coverage.
Contaminants are reduced when they contact the surface of the ISCR. This reduces the contaminant and oxidizes the ISCR. The transfer of electrons occurs at the surface of the ISCR. In the case of chlorinated ethenes, the transfer of electrons facilitates the beta elimination of two chlorines from the contaminant. In the case of trichloroethene (TCE), it is transformed by this process into chloroacetylene. Because the ISCR surface has been partially oxidized its reactivity has been lowered. One advantage of combining ISCR with ISB is that the fermenting carbon substrate will donate electrons to the surface of the ISCR particles which will re-reduce the surface and maximize the reactivity.
Because contaminant reactions occur on the surface of ISCR particles, their reactivity is proportional to their surface area. The ability to make smaller particles will result in higher degradation rates. These materials are typical injected into saturated soil and their distribution is limited by the pore throat sizes. Smaller particles will transport further on average than larger particles that are preferentially filtered out by the soil.
ISCR particles can be made of ferrous oxide, ferric oxide, magnetite, hematite, maghemite, ferrous hydroxide, ferric hydroxide, goethite, akaganeite, lepidocrocite, ferroxyhyte, ferrihydrite, schwertmannite, green rust, fougerite, iron sulfide, troilite, greigite, pyrrhotite, mackinawite, marcasite, pyrite, siderite, vivianite, iron, zero valent iron, zero valent zinc, zero valent aluminum, iron gluconate, cysteine, silver nitrate, iron sulfate, iron chloride, and iron lactate.
As the biological mechanisms of anaerobic metabolism became better understood, it became apparent that many degradation pathways could not be easily attributable to strictly biological processes, but fit easily into chemical reductive processes. Data from some sites where there is evidence of both processes suggest that there may even be a synergistic effect from biotic and abiotic processes.
The current paradigm considers combining the benefits of chemical reduction using ISCRs with reductive dechlorination through the addition of a carbon substrate. The problem for practitioners is how to mix an insoluble ISCR with a soluble organic substrate to form a stable injectate. The use of microemulsion technology to biologically enhance reductive dechlorination is a relatively new technology to the remediation industry. Microemulsions combine two immiscible materials with surfactants to form sub-micron-sized particles that are thermodynamically stable. Since microemulsions are stable systems with sub-micron particles, they exhibit superior subsurface transport. A significant advantage would be achieved if ISCRs could be incorporated into a microemulsion thereby combining both biological and abiotic mechanisms into one easily managed material that incorporates both biotic and abiotic remedial properties with the distribution characteristics of a liquid that transports in ground water like a miscible liquid.